Genes Discovered Enabling Tomatoes To Resist Wilt Disease

Rehovot, Israel -- April 23, 1997 --A group of genes that makes tomatoes
resistant
to wilt disease has been discovered in an Israeli-American study headed by Prof.
Robert Fluhr
of the Weizmann Institute of Science. The finding, reported in the current
issue of
The Plant Cell (Volume 9, No. 4, pp. 521C532), is expected to speed up the
breeding of new
disease-resistant varieties and reduce the need for chemical spraying.

Wild tomatoes -- but not cultivated varieties -- are naturally
resistant to wilt disease
spread by the soil-borne Fusarium oxysporum fungus (lycopersici strain). Once a
widespread
blight plaguing tomato crops, Fusarium wilt disease was largely brought under
control
through classical breeding techniques. However, breeding new varieties through
this method
is a laborious process that can take years because it entails producing many
successive
generations of hybrid plants in which the undesirable traits have been
eliminated
but the beneficial ones retained.

The new study facilitates this process by providing molecular markers
allowing breeders to
simply trace the genetic makeup of new hybrids rather than test for it at each
stage. The
findings also pave the way for genetically engineered new varieties with an
optimal mixture of
desired properties.

"In the absence of genes that endow plants with natural resistance to
disorders such as wilt
disease, farmers must rely on extensive chemical spraying and soil treatment,"
says Prof. Fluhr,
a member of the Weizmann Institute's Plant Genetics Department.

Collaborating with Prof. Fluhr were departmental colleague Dr. Dvora Aviv
and doctoral
student Naomi Ori, along with Prof. Dani Zamir and doctoral student Yuval Eshed
of the
Hebrew University of Jerusalem's Faculty of Agriculture and Prof. Steve Tanksley
of Cornell
University.

The newly identified genes are also of primary importance because they form
the plant's first
line of defense against Fusarium attack. Acting as regulatory switches, they
induce the release of various defensive agents such as destructive oxygen
molecules that bombard
the disease-causing organism, or enzymes that chew away at it.

"A better understanding of how the resistance genes work should allow us to
turn on these
defense components at will," Fluhr says.

"Moreover," he adds, "the methodology we have helped develop for isolating
resistance
genes should make it possible to design new resistance genes and transfer them
to various crops
where they are lacking."

Plant versus Fungus Fusarium wilt disease is caused by a fungus that
penetrates the plant
roots, primarily through wounded tissue. The fungus works its way into the
plant's vascular
system -- the vital passageway for water and nutrients -- where it thrives. Not
only does the
Fusarium fungus have a hearty appetite and consume the plant's nourishment, but
it also
produces toxins. The presence of abundant fungus, therefore, leads to the
functional collapse
of the vascular system, systemic wilting and often death of the plant.

In the study, the scientists first determined the chromosomal location of
the wilt resistance such as one
endowing tobacco leaves with resistance to viruses and another that makes the
leaves of a weed resistant to bacteria. This resemblance is intriguing since
the plant's
mechanism for defending the vascular system is very different from the one used
for defending
leaves.

When a leaf is invaded, the plant can choke off the precise area under
siege, killing infected
tissue along with the infiltrator but leaving the rest of the plant intact. But
when countering an
attack on the vascular system -- its essential lifeline -- the plant must employ
more precise
strategies, such as producing special cellular structures that confine the
fungus to the lower part
of the root and thus localize the disease.

"Not only do the products of diverse plant resistance genes share
structural features," says
Fluhr, "but one such feature, resembling a component of biological receptors,
can be
found in animals as well. This underscores the unity of the plant and animal
kingdoms."

The study was supported by the United States-Israel Binational Agricultural
Research and
Development Fund, the Israel Ministry of Science, the Commission of the European
Communities and the Leo and Julia Forchheimer Center for Molecular Genetics at
the Weizmann
Institute.

Prof. Fluhr is the coordinator of the Israel Ministry of Science's Plant
Genome Center at
the Weizmann Institute, which involves scientists from several major research
institutions in
Israel. The Center is aimed at putting Israel on the map of plant genome efforts
worldwide,
primarily by providing technology for map-based cloning and isolation of genes.

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